Improving the surface state of a cavity, a tube, a bore, or a hole machined in a mechanical part, and putting said surface into compression are known to the person skilled in the art. For example, it is possible to achieve these objectives by blasting with sand or with corundum. However, the effects of those methods are limited and it is more common practice to blast with shot, using balls of steel, glass, or ceramic. Those known methods are ineffective for treating certain surfaces, in particular the walls of a cavity of small dimensions, and more particularly the walls of a machined hole, bore, or tube of longitudinal dimensions that are much greater than its transverse dimensions. Ordinary shot-blasting by firing balls through nozzles pointing towards the part to be treated give no results on the walls of a cavity as defined above, given that the only way the balls can travel inside the cavity is along a direction that is substantially parallel to the walls that are to be treated. Since the balls do not strike the walls, or at least not hard enough to provide effective surface treatment, that method cannot be used successfully under such circumstances.
Such a situation arises, in particular, in the field of manufacturing turbines, and more particularly when mounting turbine blades in the disk of a rotor. Mounting is performed by using pins to hold together two interfitting combs, i.e. the comb of the blade and the comb of the disk are held together by two or three fixing pins that pass right through the interfitting combs.
Normal blade mounting consists in independently providing initial holes of smaller diameter than the final holes both in the blade and in the disk, prior to the blade being installed on the disk. Each hole is machined to size by means of a tool such as a reamer, for example. However, the surface state of each hole is of mediocre quality, and because the tool work hardens the metal around the hole tangentially, it sets up residual traction stresses that give rise to high sensitivity to cracking under stress and to increased sensitivity when corrosion is possible. That is why it is essential to improve the surface state, particularly whenever it is necessary for the mounting to be highly reliable.